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General information

Influenza, more commonly known as the flu, is a respiratory illness, affecting the lungs and airways, and is the result of an infection caused by an influenza virus.
There are three types of influenza virus, A, B and C. Influenza A, of which there is many strains, is found in animals, birds and humans, whereas Influenzas B and C are only found in man. Influenza C is relatively uncommon and rarely causes infection. It is also generally the case that infections caused by Influenza B are less severe than those caused by Influenza A.
Influenza A virionsNew influenza A viruses may emerge when the two different surface proteins of the influenza A virus, haemaglutinin (H) and neuraminidase (N), undergo minor progressive changes (antigenic drift) from season to season or a spontaneous and unpredictable major change (antigenic shift). Therefore influenza A is also classified into a subtype e.g. H1N1 depending on what particular change each protein has undergone.
Most people recover from a bout of flu uneventfully but complications such as pneumonia can occur particularly in the elderly or in those with certain medical conditions. These can result in serious illness and can be life-threatening.
People in groups most at risk of complications may require medical attention earlier. GPs may prescribe antiviral medications such as zanamivir and oseltamivir to such patients for both the treatment and prevention of flu, following the guidelines produced by the National Institute of Heath and Clinical Excellence (NICE). More details about these guidelines for the use of antiviral medication for the prevention and treatment of flu are given on the NICE website.
A new vaccine is developed for each flu season and offers protection against the flu strains predicted to be in circulation. Vaccination with the 'flu jab' is offered free to the elderly, people considered more at risk of developing complications, unpaid carers and those in certain residential homes.
As influenza circulates each year in the UK during the winter months (October to April), it is often called seasonal flu. Flu activity can vary from year to year and can occasionally be at epidemic activity. An epidemic is the occurrence of cases of illness in a community or region in excess of what might normally be expected. The community or region, and the period in which the cases occur are specified precisely.
Pandemics (an epidemic occurring over a very wide area and usually affecting a large portion of the population) of influenza are triggered by antigenic shift of the influenza A virus. These occur sporadically and lead to the development of new subtypes of the influenza virus. As most people have not had the chance to become immune to these new strains, widespread infection may occur. The influenza B and C viruses appear to be more stable.


Seasonal influenza: Influenza rapidly spreads around the world in seasonal epidemics and imposes a considerable economic burden in the form of hospital and other health care costs and lost productivity. Flu activity in both hemispheres peak during the winter months.
According to the World Health Organisation (WHO) in annual influenza epidemics 5-15% of the population are affected with upper respiratory tract infections. Hospitalization and deaths mainly occur in high-risk groups (elderly, chronically ill). Although difficult to assess, these annual epidemics are thought to result in between three and five million cases of severe illness and between 250 000 and 500 000 deaths every year around the world. Most deaths currently associated with influenza in industrialized countries occur among the elderly over 65 years of age.
Most cases in the UK tend to occur during a six to eight-week period during the winter. The timing, extent and severity of this 'seasonal' influenza can all vary. Influenza A viruses cause outbreaks most years and these viruses are the usual cause of epidemics with large epidemics occuring intermittently.
In the UK, influenza activity was modest during the eight influenza seasons from 2000-01 to 2007-08 compared with 1996-7, 1998-99 and 1999-2000. Severe epidemics were recorded in 1975-76 and 1989-90, resulting in an estimated 29,646 and 23,046 deaths respectively in England and Wales. Even in winters when the incidence is low, 3000-4000 deaths have been attributed to influenza. More detailed information and references for the epidemiology of influenza in the UK is given in the Public Health England 'Immunisation Against Infectious Diseases' policy and guidance handbook (The 'Green Book') for health professionals which is available from the website at:
Influenza activity in Wales was higher than the average seasonal activity in seasons 1988/9, 1990/91, 1996/7 and 1999/2000 and reached epidemic activity in season 1989/90. Influenza A was in circulation in 1999/2000 and although subtyping was not undertaken in Wales, the subtype H3N2 was found to be in circulation throughout the European countries (including England and Scotland) participating in the European Influenza Surveillance Scheme (EISS) [In September 2008, the responsibility for the former activities of the European Influenza Surveillance Scheme (EISS) was transferred to the European Centre for Disease Prevention and Control (ECDC)].
Virological testing in Wales commenced in the 2003/4 season. In that season influenza A was identified as circulating in Wales. In 2004/5 both influenza A and B were in circulation in Wales. In 2005/6 the majority of cases were of influenza B. Flu activity was also low and remained within baseline levels during the 2006/7 season. Virological testing established that influenza A was the predominant circulating strain.
During the 2007/8 season, influenza activity in Wales was again low and remained within baseline levels. However, this season saw two distinct peaks of activity due to the circulation of both influenza A (earlier in the season and peaking at 8.5 consultations per 100,000 population during week 1 2008) and influenza B (later in the season and peaking at 7.4 consultations per 100,000 population during week 16 2008). 
Oseltamivir-resistant strains of influenza A(H1N1) were also identified in circulation during the 2007/8 season from 22 European countries (overall European average percentage of resistant samples was 24%). In England and Wales, the Health Protection Agency reported 11% (38/347) of A(H1N1) tested viruses were resistant to oseltamivir; one of these resistant samples was from a patient from Wales. These viruses retain sensitivity to zanamivir, amantadine and rimantadine. [see news story-Oseltamivir resistance in flu viruses-an update]. 
The 2008-9 influenza season was dominated by influenza A, with H3 being the predominant subtype in Wales and also in Europe. The peak in activity in Wales was recorded during week 1 2009 with 24.81 consultations per 100,000 population (just below the baseline of 25 consultations per 100,000).
2009 saw the emergence of a novel strain of influenza A H1N1 (swine influenza) which led to the World Health Organisation (WHO) declaring the first global flu pandemic in over 30 years. More details about the 2009 flu pandemic are given in the sections below.
The influenza viruses in circulation during the 2010-11 season in Wales were predominantly the 2009 pandemic strain A(H1N1) with smaller numbers of influenza B cases. The peak of activity was observed during week 1 2011 at 93 consultations per 100,000 population (and just below the threshold for higher than average seasonal activity)-this is considered to be the third wave of activity of the pandemic A(H1N1) 2009 virus strain in Wales.
Influenza activity appeared low during the 2011/12 season although limited activity observed late in the season resulted in a a number of outbreaks of A(H3N2), primarily affecting elderly residents in residential or care homes.
Weekly influenza activity reports for Wales published by CDSC Public Health Wales from 2008 until now are available to download from the webpage: Weekly Influenza Activity Report
More information about the types and sybtypes of influenza in circulation in countries participating in EISS from seasons 1997/8 to the current one can be found on the ECDC European Influenza Surveillance Network (EISN) webpages.
Pandemic influenza:  Three times in the 20th century the influenza A viruses have undergone major genetic changes mainly in their H-component, resulting in global pandemics and large tolls in terms of both disease and deaths.
'Spanish Flu' hospital in USA:1918-1919 (courtsey of US National Museum of Health and Medicine)The most infamous pandemic was “Spanish Flu” which affected large parts of the world population and is thought to have killed at least 40 million people in 1918-1919. More recently, two other influenza A pandemics occurred in 1957 (“Asian influenza”) and 1968 (“Hong Kong influenza”) and caused significant morbidity and mortality globally.
In contrast to current influenza epidemics, these pandemics were associated with severe outcomes also among healthy younger persons; during the pandemic of 1918-19, the death rate was highest among healthy young adults.
On 11 June 2009, the WHO declared that a global flu pandemic was underway due to the rapid and sustained spread of swine influenza A(H1N1) across the globe from when it was first reported in Mexico in April 2009. The swine influenza A(H1N1) virus had never been identified as a cause of infections in people before the 2009 pandemic. Genetic analyses of this virus have shown that it originated from animal influenza viruses and is unrelated to the human seasonal H1N1 viruses that have been in general circulation among people since 1977. More information about the first flu pandemic of the 21st century is given in the following section.
2009 Influenza Pandemic: Human cases of swine influenza A (H1N1) virus infection were first detected in the United States and Mexico in April 2009. The outbreak spread globally and because of this the World Health Organisation (WHO) raised the pandemic alert level on June 11 2009 to WHO Phase 6. This phase is characterised by sustained human-to-human spread of the virus into at least two WHO regions. During the pandemic cases were reported worldwide from more than 214 countries and overseas territories. The pandemic extended into 2010, and was formally declared over by the WHO on 11 August 2010.
Most infected individuals reported mild illness and fully recovered but more severe illness and over 18000 deaths were reported globally during the pandemic Of these 457 deaths occurred in the UK; 342 in England, 69 in Scotland, 28 in Wales and 18 in Northern Ireland. Swine influenza can be treated with the antivirals oseltamavir (Tamiflu®) and zanamivir (Relenza®). A vaccine against swine flu is also available; the swine flu vaccination programme began on 21 October 2009 in the UK for people in certain at-risk groups.
More detailed information about the 2009 flu pandemic including surveillance data and reports, information released for health professionals and the public and media updates is available from the web page: Flu Pandemic 2009

Weekly surveillance of influenza in Wales

Public Health Wales release a weekly influenza report which is available for both health professionals and the public and is posted on this website. Reports detail rates of influenza circulating in Wales including geographical and age range distributions and results of virological testing of submitted swab samples. More information about how the data contained in these reports are collected is given in the section: Sources of surveillance data in Wales. A summary report for each flu season is also published and available.
Reports for this season are available from: Influenza activity in Wales: Weekly surveillance reporting. A new report is published every Wednesday. Reports for previous seasons are also available from this page. 
Reports from Wales are also sent to the Health Protection Agency (HPA) for inclusion in the national influenza weekly reports for the UK and to the WHO EuroFlu surveillance scheme.
Additional trend data for influenza activity in Wales from 1987-2012 is given in the section below.

Influenza in Wales: 1987-2012

Flu activity is measured on the basis of the number of people for every 100,000 population who consult their GP with flu-like illness each week. In Wales, flu activity is defined as:
  • baseline (low level) activity: less than 25 consultations per 100,000 people in the population  
  • normal seasonal activity: 25 -100 consultations per 100,000 people in the population
  • higher than normal seasonal activity: 100 -400 consultations per 100,000 people in the population
  • epidemic activity: more than 400 consultations per 100,000 people in the population
The number of cases of seasonal flu varies from year-to-year depending on the types of flu virus in circulation and the uptake and effectiveness of the vaccine. In most years, the number of cases of flu peaks between December and March.
Influenza activity in Wales: 1987-2012
weekly influenza consultations rate in Wales from GP surveillance scheme: 1987-2011
Source: Welsh GP Surveillance Scheme, Public Health Wales Communicable Disease Surveillance Centre
Week of peak rate of flu activity from 1987-2012 and predominant flu type
Flu Season (Oct-Apr)
Highest Rate
Week (Week Beginning)
Predominant Flu Type
12 (16/03/88) 
49 (30/11/89)
50 (13/12/90)
07 (13/02/91)
02 (08/01/92)
A and B
14 (31/03/93)
47 (17/11/93)
05 (01/02/95)
51 (20/12/95)
A and B
01 (12/01/97)
51 (28/12/97)
A and B
01 (10/01/99)
01 (09/01/00)
A and B
10 (11/03/01)
06 (10/02/02)
53 (05/01/03)
47 (30/11/03)
A and B
07 (20/02/05)
06 (12/02/06)
07 (18/02/07)
A and B
01 (01/01/08)
01 (04/01/09)
2009 pandemic (May 2009-August 2010)
78.6 (first wave)
66.2 (second wave)
30 (02/08/09)
42 (18/10/09)
A(H1N1) swine flu
A(H1N1) swine flu


92.8 (third wave)

01 (09/01/11)

A(H1N1) swine flu



08 (20/02/12)


Source: Welsh GP Surveillance Scheme, Public Health Wales Communicable Disease Surveillance Centre

Sources of surveillance data in Wales

In recent years, enhanced influenza surveillance during the flu season has been carried out in Wales through the participation of sentinel GP practices who participate in the GP Surveillance Scheme and are located throughout Wales. GPs from these practices are asked to record the number of cases of certain infectious diseases they diagnose each week (including flu) and take nose or throat swabs from some patients who consult them with flu-like illnesses. Until 2010 this surveillance was only carried out during the flu season from week 40 until week 20 of the following year. However, enhanced surveillance is now undertaken throughout the whole year. 
These swabs then undergo molecular testing at the Public Health Wales Specialist Virology Centre. The pathogens tested for are: influenza A, influenza B, respiratory syncitial virus (RSV), parainfluenza, rhinovirus, mycoplasma, human metapneumovirus and adenovirus.  
The data collected is compiled into weekly influenza reports which are available for both health professionals and the public and are posted on this website. From the 2010/11 season, surveillance data from these sentinal practices is collected using the Audit+ Data Quality System, more details of which is given on the webpage: Flu Pandemic 2009: Surveillance Systems for Wales
This enhanced influenza surveillance enables levels of circulating flu activity in the community and the strains of flu virus in circulation to be established. This information is used to aid decision-making with regard to the use of antivirals and enables predictions on hospital emergency bed requirements to be made.
Rates of influenza-like illness in GP practices participating in the surveillance scheme are reported throughout the year and published in the weekly GP surveillance newsletter which can be viewed from the link: GP Surveillance Newsletter

Vaccination against influenza

In Wales and the UK, a vaccine (the 'flu jab') is routinely offered to people through their GP who are considered to be more at risk of developing complications from contracting flu. These include those:
  • aged 65 and over
  • living in long-term residential or nursing homes
  • with chronic heart or chest complaint, including asthma
  • with chronic kidney or liver disease
  • with diabetes
  • with cerbrovascular disease (principally stroke and transient ischaemic attacks (TIAs))
  • with multiple sclerosis and related conditions
  • with a hereditary and degenerative disease of the central nervous system
  • with lowered immunity due to disease or treatment such as steroid medication or cancer treatment
  • those who are the main carer for an elderly or disabled person whose welfare may be at risk if the carer falls ill
  • who are pregnant 

Additionally, arrangements are made to offer vaccination to health and social care workers. 

A new vaccine is developed for each flu season and offers protection against the flu strains predicted to be in circulation.
The World Health Organisation (WHO) identifies the strains that are likely to most resemble those in circulation during the next year's winter seasons and then recommends the composition of the vaccine.  Each yearl the vaccine comprises two influenza A strains and one influenza B strain. The compostition of the vaccine for the 2012/13 northern hemisphere flu season is available from the WHO website at:
The vaccine are 70-80% effective in healthy adults in years when there is a good match between the vaccine and the strains of flu in circulation. People who catch flu despite being vaccinated will usually have a milder illness.
Reports on the rate of uptake of flu vaccination in Wales can be found by following the link: Influenza Immunisation Uptake in Wales
More information about the composition of vaccines including the influenza vaccine is included in the Public Health England 'Immunisation Against Infectious Diseases' policy and guidance handbook (The 'Green Book') for health professionals which is available from the website at:

Requests for further surveillance data

If further surveillance data for influenza in Wales is required, it may be possible to provide it on special request. Please use the surveillance data request form provided from this link.

Links to other influenza surveillance

Last updated: 09/04/2013